In electric power networks for transmission and distribution of alternating current, the voltage tends to vary, primarily because of variations in the current in the power network. Power transformers in such power networks are therefore often equipped with a tap-changer, which permits a stepwise switching of the ratio of the transformer under load. To this end, such a transformer has a main winding on each of its primary and secondary sides and, for example, on its primary side at least one and normally two control windings with different numbers of winding turns, each one with two terminals. By connecting these control windings in such a way that the voltage between their respective terminals either contributes to or counteracts the voltage between the terminals of the primary main winding, the transformer ratio may, with a few switching operations, be changed in a plurality of steps of a predetermined magnitude.
The tap-changers are normally controlled by control equipment, which, in dependence on a comparison between a reference value for the voltage at a certain point in the power network and a sensed voltage at this point, forms a control signal for the tap-changer, which is supplied to the tap-changer for ordering switchings of the control windings.
The voltage of the power network comprises, in addition to a component of the fundamental frequency, usually also components of higher frequencies, harmonics, generated, for example, by pieces of equipment connected to the power network. These harmonics do not necessarily have the same phase position as the component of fundamental frequency. Voltage drops in the power network are usually for the most part related to the fundamental tone, since harmonics and other disturbances generally emanate from equipment connected to the power network.
Motors, for example, may be subjected to harmful overheating in the event that the root mean square (rms) value of their supply voltage exceeds a predetermined value, whereas other types of equipment, for example computer equipment, usually are more sensitive to the peak value of the voltage. Since the voltage, in addition to the fundamental component, also contains harmonics, its peak and rms values do not generally correspond in such a way that it is possible to simultaneously maintain both the peak and the rms values separately at the desired values.
The fundamental frequency, usually nominally equal to 50 or 60 Hz, is generally not fixed but varies in dependence on a plurality of factors. In strong power networks with good frequency control, the frequency variations typically amount to .+-.0.1 Hz whereas in extreme cases they be of the order of magnitude of .+-.5 Hz.
Up to now, the above-mentioned switching operations in the tap-changer have usually taken place by means of mechanical switching devices, which, among other things, has resulted in the switching time from one tap-changer position to another being relatively long, and hence also the requirements for speed of the control equipment have been relatively low.
US patent document U.S. Pat. No. 4,419,619 discloses a transformer device of the kind described above as well as control equipment for its tap-changer composed around a microprocessor. The secondary voltage is sampled on a number of sampling occasions and the sampled values are supplied to the microprocessor which is adapted, by means of the so-called Discrete Fourier Transform (DFT), to convert the sampled voltage values into a digital signal corresponding to the RMS value over a period of the sensed voltage. A difference of a reference value, also generated in digital form, and the mentioned digital signal corresponding to the RMS value of the sensed voltage is formed and supplied, after conversion into analog form, to a motor-driven switching device with rotating contacts for executing switchings of the control winding of the tap-changer.
The secondary voltage is sampled at 16 sampling occasions for a cycle of the voltage and the data collection, in order to form a mean value, takes place during 32 cycles of the fundamental tone. As reference point for starting the sampling cycle, a zero crossing for the secondary current of the transformer is used. After 32 cycles, the processing of the 16 * 32 sampled values is started, apparently first by the formation of a mean value and then by means of the above-mentioned Fourier transform. The processing results in a determination of a measure of the RMS value of the secondary voltage over a cycle of the fundamental tone.
The Fourier analysis is carried out only for the fundamental component, but it is indicated that the algorithm may be modified to analyze also tones of a higher order, which according to this document would be of particular interest in applications comprising thyristor-switched capacitors.
The algorithm for the discrete Fourier transform may be conceived as a selective filter which, from the sequence of sampled values, determines and forwards values of amplitude and phase angle for a component in the sequence of a predetermined selecting frequency. In the device described in the cited patent document, the selecting frequency is equal to the nominal frequency of the power network, that is, it is assumed that the frequency of the fundamental component is equal to the nominal frequency of the power network.
US patent document U.S. Pat. No. 5,581,173 describes a similar use of a microprocessor, also with 16 samples per cycle of the fundamental tone. In this case, the sampled values are sensed on a half-wave-rectified signal corresponding to the secondary voltage of the transformer device, and therefore half of the sampled values are equal to zero. In the device described in this document, a mean-value formation is performed over 8 periods of the secondary voltage of the transformer device.